This invention relates generally to interconnected networks of computers and related devices and, more particularly, to techniques facilitating communication between stations connected to widely separated local area networks (LANs). A local area network includes a set of stations or nodes connected to a common communication bus or ring. Typically, all elements of a-single LAN are located in a single building or group of buildings. Multiple LANs are often connected together by devices called bridges, to form an extended LAN. A bridge is a device that is connected to more than one LAN, "listens" to message traffic on each of its LANs, and forwards the traffic onto selected other LANs. The bridge message forwarding operation is limited by a spanning tree algorithm in which all bridges participate. Messages are forwarded only over a loop-free spanning tree, to avoid closed loops and multiple copies of messages. Message forwarding is further controlled by the bridge's "learning" function. Each bridge learns the directional locations of various stations that are the sources of messages it hears, and "remembers" these locations for future use in forwarding message traffic.
Multiple LANs and extended LANs may also be interconnected to form wide area networks (WANs). The mechanism through which extended LANs are interconnected is typically through devices such as routers. At a broad conceptual level, routers perform a similar task to bridges in that each router is connected to multiple LANs or extended LANs. However, the router operates at a different protocol layer (known as the network layer), and a router can be used for forwarding traffic only if the source of the traffic cooperates by using a network layer protocol that is understood by the router. A bridge, on the other hand, is a packet switch that is "transparent." A station that does not have a network layer protocol, or has a network layer protocol that is not implemented by the router, can use the bridge as a packet switch. Unfortunately, however, bridges have significant drawbacks that preclude their use in many contexts.
The most significant drawback of a bridge, as compared with a router, is that bridges use only a subset of the actual topology of the interconnected networks. In particular, redundant paths cannot be used, because to do so would result in closed loops and unwanted multiplication of message traffic. As already mentioned, to ensure a loop-free topology bridges participate in a spanning tree algorithm, which establishes a loop-free tree structure. If two extended LANs were to be interconnected by a bridge, the separate spanning trees of the two networks would have to be combined, forming one larger extended LAN. This might not be desirable from a network administration standpoint. Another important disadvantage of bridges is that they cannot forward traffic through a router, because the packets handled by the bridge do not necessarily have the appropriate network layer information needed to utilize the router. Also, since routers do not participate in the spanning tree algorithm, there is a possibility of forming loops in the topology if bridges were to forward traffic through routers.
In summary, neither routers nor bridges provide an ideal solution to the problems of forwarding message traffic over wide-area networks. Some communication protocols cannot work through routers, since routers support only certain network layer protocols. But bridges are limited, by the spanning tree algorithm, to a subset of the overall network topology, and cannot be used to forward traffic through routers that separate LANs. Application Ser. No. 07/489,910, filed Mar. 7, 1990, entitled "Utilization of Redundant Links in Bridged Networks," now U.S. Pat. No. 5,150,360, provides for the establishment of point-to-point links outside the spanning tree, but still does not permit forwarding of traffic through traditional routers.
What is needed is a new approach that permits traffic to be forwarded transparently across WANs, using more optimal routes and permitting the traffic to traverse traditional routers. The present invention satisfies this need.
Another way of viewing the problem that this invention solves is that, ideally, there should be some way to address messages directly to particular extended LANs. However, the addressing conventions that have evolved for interconnected networks do not include any convenient means for addressing extended LANs. What is needed, then is some way of forwarding a message more directly to the extended LAN in which the message destination is located. The present invention also satisfies this need.